This invention relates to an open switching interval protection circuit, and, more particularly, to a circuit for guarding against premature release of key system holding conditions due to central office network reconfigurations.
There are, today, approximately a million telephone stations installed on customer premises which operate with an older type of key telephone system. This type of key telephone system was originally installed at a time when telephone central offices were all of the electromechanical type. With the advent of electronic central office switching systems, such as the No. 1 ESS manufactured by Western Electric Company, a new type of telephone office switching operation came into use. In the ESS type of switching system employing ferreed crosspoints, battery may temporarily be removed from the tip and ring conductors during switching network reconfigurations. The period of time during which battery may so be removed from the line is called the open switching intervaL (OSI) and may last up to 460 milliseconds. At the customer's premises, the key telephone system of the older type, known as the 1A1 key telephone system, may be sufficiently sensitive to the OSI to inadvertently disconnect or drop a holding condition that had been placed on the line by the customer.
Heretofore, a number of solutions have been proposed to prevent the 1A1 key telephone system from inadvertently dropping held calls when served by an electronic central office. For example, the newer type line circuits, such as the 400 type circuit manufactured by Western Electric Company, include circuitry for maintaining the holding bridge connection during the OSI. This line circuit disclosed in Limiero U.S. Pat. No. 3,840,710, issued Oct. 8, 1974, contains a delay circuit that prevents the release of the holding bridge for a certain period of time when the line current is interrupted. The time interval is predeterminable by strap connections accessible on the line circuit terminal strip.
Another approach is shown in Angner et al U.S. Pat. No. 3,895,192, issued July 15, 1975, which contains a circuit for timing the release of the holding bridge when the line current is interrupted. This arrangement uses a time delay interval controlled by a capacitor discharge circuit.
Still another approach is shown in Lancaster U.S. Pat. No. 3,996,426, issued Dec. 7, 1976. This arrangement involves the switching-in of an auxiliary 48-volt battery supply directly across the tip and ring leads to replace the central office battery that is removed during the OSI. Since the operation of the detector circuitry of the Lancaster arrangement depends on its direct connection across the tip and ring of the line, special circuitry is required to inhibit its operation during ringing and during power cross testing which is performed by the electronic central office switching system on every call origination. Because the Lancaster arrangement requires 48-volt battery which is not usually available in the key telephone systems located at customer's premises, the Lancaster arrangement is suitable for use only at the central office. In addition, because of leakage current that may be present between the tip and ring leads and ground, the bias of the detectors used in the Lancaster patent circuit may be affected and lead to unreliable operation.
Other prior art approaches to the solution of the problem that involve alteration to the ferreed contacts to withstand continuous line current during switching or the reprogramming of the switching operation are discussed in the Lancaster patent.